In gasoline direct injection (GDI) systems fuel may be injected at relatively higher pressures over a common rail system directly into the combustion chamber of an engine. Gasoline direct injection allows for stratified charge, wherein the air/fuel mixture may be layered. A rich portion of the air to fuel mixture may be directed around the spark plug and fresh air or a mix having a relatively lower air to fuel ratio may be present around the richer portion. In addition, GDI systems may be run at relatively lean conditions, where air to fuel ratios may be 50:1 or higher. However, running GDI systems at lean conditions may require relatively costly after treatment systems or may fail in attaining various emissions standards.
Exhaust gas recirculation may provide benefits in terms of reducing certain emissions and improved fuel consumption at moderate loads, including those greater than 5 bar and up to 8 bar. Such benefits may include improvements in fuel consumption, carbon monoxide emissions and nitrous oxide emissions. For example, improvements in fuel consumption for a given engine may be in the range of 1% to 3%, nitrous oxide emissions may be reduced by 10% to 80% and carbon monoxide emissions may be reduced by 5% to 20% upon the introduction of 5% to 20% EGR at 1500 rpm and 8 bar. In addition, the coefficient of variation of the indicated mean effective pressure (cov imep) may be less than 1.5%. At these conditions, some increase in hydrocarbons may be exhibited, in the range of under 5% to 45%.
However, this may not be true across all engine speeds and loads, such as low loads of 5 bar and less, as increased EGR amounts may lead to engine instability as reflected by an increase in cov imep. For example, the cov imep at an engine speed of 2,000 rpm and pressures of 2 bar and 5 bar may be greater than 1% and up to 6%. While some improvements in fuel consumption may be seen up to 3% some decreases may also be exhibited at loads of 5 bar.